The invention provides a piezoelectric actuator, e.g., for actuating a mechanical component such as a valve or the like, according to the generic features of the primary claim.
It is generally known that, by utilizing the xe2x80x9cpiezoelectric effectxe2x80x9d, a piezoelectric element can be built using a material having a suitable crystal structure. When an external electrical voltage is applied, a mechanical reaction of the piezoelectric element takes place that represents a push or a pull in a specifiable direction depending on the crystal structure and the application site of the electrical voltage.
The piezoelectric actuators named previously are often used to position valves. In this case it should be noted, among other things, that their lifting ability for actuating a valve tappet, for example, is relatively small. On the other hand, the different thermal expansion of the ceramic of the piezoelectric element and the housing leads to problems; the piezoelectric element has a very low temperature expansion and the housing, which is usually metallic, has a positive temperature expansion, which can lead to a drifting of the position of the valve tappet without control by the piezoelectric element.
So far, such perturbing actions could only be diminished in the usual fashion by using very expensive materials, such as invar, which have a negative temperature expansion. Another method was to connect a material with high temperature expansion in series with the piezoelectric element, which, however, reduces the stiffness of the system and, therefore, the acutator force.
The piezoelectric actuator already explained hereinabove, which can be used to actuate a mechanical component, for example, advantageously comprises a piezoelectric element with which, according to the invention, a compensating element is arranged in parallel. It is particularly advantageous thereby that the piezoelectric element and the compensating element basically have the same temperature expansion coefficients, so that the temperature-induced expansions of the piezoelectric element and the compensating elementxe2x80x94when the two elements are mechanically installed in suitable fashionxe2x80x94cancel each other out in the effective direction in such a fashion that an actuating element solidly connected to a pressing plate of the piezoelectric element remains in its position. It can therefore be achieved in simple fashion that a metallic housing, e.g., made of steel, is still used for the piezoelectric actuator, and the piezoelectric element can be fastened in the housing in such a fashion that the compensating element for temperature compensation is always solidly connected to the piezoelectric element. In an especially preferred exemplary embodiment of the invention, a heat transfer compound is located between the piezoelectric element and the compensating element, with which a good temperature compensation between the compensating element and the piezoelectric element can be produced.
According to the invention and in advantageous fashion, the piezoelectric element can be pretensioned in itself in simple fashion. The piezoelectric element is supported on one end against a fixed support plate that bears against a housing for the piezoelectric actuator via a spring. At the other end, the piezoelectric element is connected via a pressing plate to a pretensioning spring which, in turn, is held with its other end against the fixed support plate. The compensating element thereby lies basically parallel to the piezoelectric element and also bears against the fixed support plate at one end; at the other end, the compensating element solidly abuts the housing. The pretensioning spring and the piezoelectric element can thereby be arranged in simple fashion in tandem, for example, whereby the movable end of the piezoelectric element can be connected to the pressing plate via a tightening strap. It is also possible, however, that the pretensioning spring and the piezoelectric element are situated parallel to each other, whereby, for instance, two symmetrically located zigzag springs can be located in the heat transfer compound as pretensioning springs that are also parallel to each other.
The proposed interconnection of the piezoelectric element, the ceramic ring, and the pretensioning spring is solidly braced with the housing, whereby the pretensioning forces are much higher than the switching forces of the piezoelectric element, and the pretensioning spring makes a temperature compensation motion relative to the housing possible. The magnitude of the pretension can thereby be produced in simple fashion via a mounting nut. Even when the temperature expansion coefficient of the compensating element does not correspond to that of the piezoelectric element, the temperature compensation can be xe2x80x9ctunedxe2x80x9d via the length of the compensating element. In the exemplary embodiment, the support of the compensating element on the mounting nut represents the common zero point, which mounting nut is solidly connected to the housing.